This application claims priority from Japanese Application No. JP 2000-209829 filed on Jul. 11, 2000.
The present invention relates to a process for the production of a highly chemically stable hydrophilic ODS packing material made of silica gel for liquid chromatography which causes less of the phenomenon of water repellency when using an aqueous eluent.
Octadecylsilyl silica gels (ODS silica gels) are currently widely used as packing materials for high-performance liquid chromatography (HPLC).
Conventional ODS packing material made of silica gels have the defect that the retention time of the target compound for an analysis gradually reduces due to the phenomenon of water repellency, which is believed to be caused by collapse of the ODS group, which is a chemical modifier, (loss of hydrophilicity by changing into a wax) when repeating an analysis using a 100% aqueous eluent, so making it difficult to carry out a chemical analysis.
There has therefore been a strong desire for the development of a highly chemically stable packing material that does not affect the retention time of the target compound for an analysis even when using a 100% aqueous eluent. As a result of an intensive investigation for the purpose of satisfying this desire, an ODS packing material made of silica gel (hydrophilic ODS packing material made of silica gel) that serves the above-mentioned purpose has been developed and is commercially available.
The above-mentioned hydrophilic ODS packing material made of silica gel is produced by a process (end capping) involving chemically modifying the surface silanol groups of silica gel with ODS groups and then bonding short-chain alkyl groups to unreacted silanol groups (active silanol groups), which cause peak tailing and poor reproducibility when HPLC analysis is carried out. Since the active silanol groups strongly interact with, in particular, basic materials, they cause peak tailing when analyzing basic materials.
When producing a hydrophilic ODS packing material made of silica gel that can be used in a 100% aqueous system by the above-mentioned production process, the following three types of technique are employed.
(1) A technique for preventing ODS groups collapse by decreasing the degree of chemical modification of the silica gel with ODS groups so as to maintain the hydrophilicity.
(2) A technique for preventing ODS groups collapse by introducing a hydrophilic functional group so as to maintain the hydrophilicity.
(3) A technique for preventing collapse by changing the stationary phase.
However, hydrophilic ODS packing materials made of silica gel produced with these techniques have the following analytical defects.
That is to say, in the case of the above-mentioned technique (1), since a large amount of active silanol groups (ion-exchangeable silanol) remains, secondary interactions with basic materials are strong so easily causing peak tailing. Moreover, the remaining active silanol groups reduce the chemical stability of the column, particularly, in the alkaline region.
In the case of the above-mentioned technique (2), the elution characteristics (selectivity) depends to a great extent on the type of functional group selected. Furthermore, peak tailing is easily caused not only in the analysis of basic materials, but also in the analysis of acidic materials.
In the case of the above-mentioned technique (3), the collapse of chemically modified groups is prevented by changing the chemical modifying group, for example, by using a silanation agent having a longer carbon chain so as to solidify the packing material. When an organic solvent type eluent is used, the selectivity varies to a greater extent than is in the case with generally used ODS packing materials made of silica gel.
That is to say, the hydrophilic ODS packing materials made of silica gel produced by a process employing any currently available technique cannot necessarily give satisfactory performance when using an aqueous solvent.
For example, an aqueous eluent is ideally used for the HPLC analysis of a hydrophilic compound, but the decrease of the retention performance of the sample with time makes the isolation and the quantitative analysis difficult when using a conventional ODS column. Therefore, in order to prevent these phenomena, an organic solvent type eluent is often employed for the HPLC analysis of a hydrophilic compound. In this case, in order to increase the retention performance of the sample an ion pair reagent is used. Accordingly, an organic solvent is often used as a mobile phase for HPLC at the present time, and this is not desirable in terms of environmental protection.
It is an object of the present invention to provide a process for producing a highly chemically stable hydrophilic ODS packing material made of silica gel that causes hardly any peak tailing even when using an aqueous eluent, thereby eliminating the above-mentioned defects.
As a result of an intensive investigation by the present inventors in order to achieve the above-mentioned object, it has been found that, with regard to the above-mentioned technique (1), the above-mentioned defects can be eliminated by deactivating the active silanol groups of a silica gel having a smaller surface area than the silica gel used in the prior art by one- or two-step end capping involving high temperature end capping. The present invention has thus been accomplished.
The present invention relates to a process for the production of an ODS packing material made of silica gel, wherein after chemically bonding ODS groups to a silica gel having a specific surface area of 200 to 300 m2/g, the remaining active silanol groups are deactivated by high temperature end capping.
The present invention relates to the above production process of an ODS packing materials made of silica gel, which is characterized in that the high temperature end capping is carried out in a liquid such as liquid paraffin of a temperature range of 200 to 300xc2x0 C.
At present, the vapor phase method (Japanese Patent No. 2611545, JP, 4 -212058, A and U.S. Pat. No. 5,134,110, A) and the super critical fluid method (Japanese Patent No. 2818857, JP, 9-49829, A) as high temperature end capping technique were reported. Each of them shows the production methods under the special environments. The reactions according to the present invention are carried out in liquid phases, therefore are superior to the former methods also from the viewpoints of economics.
The present invention relates to the above production process of the ODS packing materials made of silica gel, which is characterized in that the high temperature end capping is carried out subsequent to ordinary end capping, which is conducted using toluene or xylene, at the reflux temperature thereof.
The present invention relates to the above production process of the ODS packing materials made of silica gel, wherein the ODS group is one group selected from the group consisting of dimethyloctadecylsilyl group, methyloctadecylsilyl group and octadecylsilyl group.
According to the hydrophilic ODS packing material made of silica gel for HPLC obtained by the production process of the present invention, even when analyzing a basic material using a hydrophilic solvent, a reliable quantitative analysis can be carried out with high chemical stability while suppressing peak tailing.